A few years ago it seemed unthinkable. But recently, public figures on both the Right and Left have called for the decriminalisation of drugs. What are they thinking?

Back in 2004, when I was living in Clapham Common, I found myself part of an experiment in the decriminalisation of drugs. Well, one drug: cannabis. And it wasn’t just me, it was all my neighbours in the London borough of Lambeth.

David Blunkett, the then home secretary, had decided to downgrade cannabis from a class B to a class C drug, the category which includes tranquillisers. This effectively meant you wouldn’t be arrested for possession of the drug in our borough.

But the experiment, intended to free-up police resources to fight hard drugs such as heroin and cocaine, came up against an unforeseen problem. The cannabis or marijuana of 2008 was not the same as the fairly mild marijuana the hippies smoked back in 1968. The street names were the same – weed, ganja, dope, hash, grass – but it was far more potent, especially the variety known as skunk, and evidence was mounting that it was causing psychosis in certain people.

Four years after the experiment began then, another Labour home secretary, Jacqui Smith, bumped cannabis back up to a class B. Now possession would get you five years in prison.

The Lambeth experiment was deemed a resounding failure. So it’s hardly surprising that, as recently as 2010, it was pretty much a sackable offence for a politician to advocate decriminalisation. That was the yearDavid Nutt, Labour’s chief adviser on drugs, found himself out of a job when he suggested that drugs should be categorised according to their harms and legislated for accordingly – and that alcohol was far more pernicious than cannabis.

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Yet in recent months there has been a shift in mood and politicians are no longer afraid to debate decriminalisation. Why? What has changed? Well, the short answer is the attitude of world leaders.

To explain why, we need to do the long answer, and start with some facts. The harms drugs can inflict are indisputable. Drug misuse is the cause of around 2,000 deaths a year in this country, and some half a million worldwide. About 330,000 people in England are dependent on heroin and/or crack cocaine. Many of these pay for their drugs through crime. It is estimated that these crimes cost us, the taxpayer, £13.9billion a year.

How, then, could legalising drugs, as its advocates argue, really be the lesser of two evils? Well, the more you delve into this subject, the more you see the ironies. Take the easy availability of drugs in prison, where inmates are supposed to be rehabilitated. Or the fact that cannabis use in Holland, where the sale of the drug is tolerated in coffee shops, is lower than in Britain, where it’s illegal (around two to three million people use it regularly in this country).

One of the problems with this debate is that when people talk about drugs, they don’t always distinguish between soft or hard drugs. To those opposed to decriminalisation, there is little difference – and we will come to their arguments. For now, let us examine the case for decriminalisation.

It starts with semiotics, rather than narcotics. George Bush wasn’t the first president to declare war on a common noun. That honour goes to his Republican predecessor, Richard Nixon, who in 1971 declared war on drugs, and said this war would result in a drug-free world. Now, more than 40 years on, there is a growing movement to declare that war unwinnable, not least because there are more drug users around than ever before – 250million worldwide according to UN estimates. And illicit drugs are now the third most valuable industry in the world, after food and oil, estimated to be worth $450billion (£294 billion) a year, all in the control of criminals.

One of the most surprising aspects of the current debate about decriminalisation is that it’s not just the usual civil liberties suspects who are prompting it. The influential right-wing American think tank The Cato Institute and the far-from-left-wing Economist magazine are both in the decriminalisation camp. And at the House of Lords a few months ago,The Beckley Foundation, an Oxford-based charitable trust which researches drug policy reform, launched a campaign calling for an end to the war against drugs.

Among the half a million signatories they have gathered so far are several past and present presidents of Latin American countries, and even one former president of the United States, Jimmy Carter. There are high-profile professors as diverse as Noam Chomsky and Niall Ferguson. And among the knights and peers are Sir Richard Branson and Sir Peregrine Worsthorne, the former Sunday Telegraph editor.

And the main reason for this change in attitude? In a word, Portugal. Ten years ago, the Portuguese government decided to decriminalise all drugs, hard and soft, and, while still prosecuting traffickers, give heroin addicts clean needles, therapy and treatment with methadone. It seems to have worked in as much as it has brought down drugs-related crime and reduced the number of younger people trying heroin. Perhaps most significantly, drug use hasn’t gone up. Partly in response to this, President Barack Obama has signalled that he is prepared to listen to the arguments for decriminalisation. A recent referendum in California, meanwhile, means that cannabis is almost certainly going to be legalised there.

Closer to home, momentum is growing for a change in approach. Writing in a new report by the think tank the International Institute for Strategic Studies, Nigel Inkster, the former assistant chief of MI6, concludes that the global war on drugs has failed and that legalisation should now be considered.

I ask Steve Rolles of Transform, a think tank which argues that drug prohibition itself is the major cause of drug-related harm, why he thinks this change in mood is happening now. “As much as anything, I think it’s to do with economics,” he says. “We’re in a recession, yet across the whole criminal justice system we are spending around £4billion a year on drug enforcement that plainly isn’t working because drug use and drug-related crime is going up. Then there is the possible tax revenue that might come from legalising and regulating cannabis to consider, about a billion pounds by some estimates.”

Counterintuitively, the most vocal advocates for a change in policy are libertarians and free market capitalists on the right of the political spectrum, with neoliberal economists arguing that if the goal is to reduce the level of harm drugs do, then they should be available to purchase safely and legally in a consumer-driven marketplace. Although he has gone quiet on the subject for the moment, when David Cameron stood for the Tory leadership, he favoured “fresh thinking and a new approach” on decriminalisation. And it is telling that he was a fan of the television series The Wire, which eloquently put the case for legalisation. Cameron’s progressive thinking, however, did not survive his move to Number 10.

One of the leading campaigners for the legalisation of drugs in this country is Misha Glenny, the author and broadcaster. He would like to see a regulated system in which all drugs are supplied safely, i.e. heroin and cocaine not cut up with rat poison, fibreglass or brick dust, and taxed like alcohol and tobacco. He says the drug barons of central and South America are desperate to keep the war on drugs going because they make more money that way. Moreover, he believes that the tens of thousands of Mexicans being killed in the trafficking of drugs there each year are only being killed because the drugs are illegal. Make them legal, legitimise the traffic and the cartels disappear and the killings end.

I ask Glenny when he thinks a change in drug policy might happen? “We’re likely to see some significant change on marijuana within the next five to 10 years,” he says. “I’ve always argued that you should legalise it separately, then you can wait for five or 10 years and see if Western civilisation collapses as a consequence. If it hasn’t, then you can start looking at the Class A narcotics as well.” But decriminalisation was tried in Lambeth, I say, and it failed because the cannabis became too strong and dangerous.

“That change in the strength of cannabis took place within an unregulated market,” he says. “The chemical THC determines potency. If the state decides not to regulate the THC levels of the drugs being sold, then what do you expect? A good comparison here is with alcohol. You will be hard pushed to find alcohol above a certain percentage in the UK because it is regulated by the state. That works well.”

Rolles thinks some of the drug war zones such as Afghanistan have brought home the futility of the drug war to politicians and the wider public alike. “The war against the Taliban doesn’t seem to have had the slightest impact on opium growth,” he tells me. “And in Mexico, 50,000 people have died from drug-related violence since 2006. If the president of Columbia, the epicentre of the war on drugs, is saying he is up for a debate on legalisation, that suggests a sea change. That’s not just [Guardian columnist] Polly Toynbee saying it, this is the highest people in the highest international forums saying it – the UN and the G20.”

But let’s not get ahead of ourselves. There are many counter arguments. One is that drugs are a special case for control and prohibition because when people become addicted to them they are no longer acting out of choice. In other words, a responsible society should protect drug addicts from themselves. Also, it seems reasonable to suppose that drug use would go up if it were decriminalised, because there would be one less reason not to try it. There needs to be a proper stigma about drugs to stop people experimenting with them.

Ann Widdecombe, the former Home Office minister, is one of the more robust advocates for a zero-tolerance approach to drugs. “There are huge arguments against decriminalisation,” she tells me. “There are only two ways of doing it, either you decriminalise all drugs or only the soft drugs. If you decriminalise just the soft drugs, all the efforts of the drug barons will then be poured into the hard drugs. Secondly, for a percentage of people, soft drugs take you through the gateway to hard drugs.” Widdecombe believes the reason we don’t win the war against drugs is that we don’t actually fight it. “We do take a strong line against the importation of strong drugs, but we do not have any stamp down at the lower end. What I propose is zero tolerance, from hard drugs down to the possession of soft drugs, obviously with hugely different penalties.”

Theodore Dalrymple is a former prison doctor who specialised in cases involving drug offenders. People, he believes, get the relationship between crime and drugs almost exactly the wrong way around. “Nearly all heroin addicts who are in prison had criminal records before they became addicts,” he tells me. “In other words, their criminality is not the result of their taking heroin. If anything, whatever attracts them to criminality also attracts them to heroin.”

What does he think is prompting the mood change about drugs at the moment? “Self indulgence. Saying the war against drugs is unwinnable is like saying the war against burglary is unwinnable and we should open our doors. Absurd. War is the wrong word. A dim-witted metaphor.” Did he feel the heroin addicts he met had a choice? “Yes, because apart from anything else, thousands and thousands do give up. Mao was the greatest drugs therapist in the world because he threatened to shoot them and, lo and behold, 20million gave up.” Do soft drugs lead to hard drugs? “There is no psychological reason why that would be the case. It might be true sociologically, though.”

So what are we to make of these arguments and counter arguments? I can at least appreciate the argument for soft drugs being legalised, as long as anyone endangering others by driving a car under the influence of them is sent to prison. As to the “gateway”, it seems to be a matter of exposure: when users buy cannabis it’s usually from dealers who are going to offer them cocaine or heroin.

The argument for legalising hard drugs? That seems even less convincing. You can’t really have moderate heroin use in the way you can tobacco or alcohol. If heroin was made legal, surely many people who wouldn’t have dreamt of trying it would give it a go. And in purely practical terms, how would this actually work? Dalrymple made a good point when I asked about this: “The idea that we could regulate drugs carefully is ludicrous,” he told me. “We can’t even regulate a public examination system in this country.”

This article also appeared in SEVEN magazine this Sunday, free with the Sunday Telegraph. Follow SEVEN on Twitter @TelegraphSeven